Alleged Sterility of Hybrids 
all the difficulties in the way of accepting the 
theory of natural selection as a complete expla- 
nation of the origin of species, has been the 
remarkable difference between varieties and 
species in respect of fertility when crossed. 
Generally speaking, it may be said that the 
varieties of any one species, however different 
they may be in external appearance, are per- 
fectly fertile when crossed, and their mongrel 
offspring are equally fertile when bred among 
themselves; while distinct species, on the other 
hand, however closely they may resemble one 
another externally, are usually infertile when 
crossed, and their hybrid offspring absolutely 
sterile. This used to be considered a fixed 
law of nature, constituting the absolute test and 
criterion of a species as distinct from a variety; 
and so long as it was believed that species were 
separate creations, or at all events had an origin 
quite distinct from that of varieties, this law could 
have no exceptions, because if any two species 
had been found to be fertile when crossed and 
their hybrid offspring to be also fertile, this fact 
would have been held to prove them to be not 
species but varieties. On the other hand, if two 
varieties had been found to be infertile, or their 
mongrel offspring to be sterile, then it would 
have been said—These are not varieties, but 
true species. Thus the old theory led inevitably 
to reasoning in a circle, and what might be 
H 113 
